The present invention relates to a defrost thermostat used in a reversible refrigerating cycle for heat pumps and more particularly to a timer driven cam arrangement for a defrost thermostat in which unnecessary initiation or repetition of defrost operation can be prevented immediately after the termination thereof.
Conventionally, the technique of removing ice or frost accumulated on the external evaporator during the heating operation in a building by means of a heat pump is disclosed in the U.S. Pat. No. 3,320,386. The automatic defrosting control according to the U.S. patent includes thermally responsive means, a timer driven cam means, and first and second block means to transmit the respective actions of the thermally responsive means and the timer-driven cam means to the switch. The operation of the defrosting control is characterized in that the defrosting operation is terminated in response to the blocking action of the first blocking means for the switching off of the defrosting operation, said blocking action being actuated by the thermally responsive means subjected to the temperature rise of the outdoor evaporator unit as a result of the defrosting operation.
In case, however, the outside temperature is below 0.degree. C., the sensor starts to act as if there were an accumulation of ice on the external evaporator even when the defrosting operation is completed. Unless, during that time, there is any change in the cam condition, the defrosting operation starts again. As a result, the indoor heating operation comes to a stop shortly after the beginning of the heating phase. In the case of a refrigerator which is required to maintain the indoor temperature usually below 0.degree. C., there is a disclosure contained by Japanese Publication No. 40-23105 that the cam recess is blocked immediately after the cam follower rises in the cam recess due to an increase of temperature in the external evaporator to prevent the restart of defrosting cycles.
However, such a particular care has not been taken in the field of heat pump which are used under conditions in which the temperature around the evaporator does not always fall below 0.degree. C.